Field of the Invention
Embodiments of the present invention relate in general to cathodic protection and specifically to internal cathodic protection of a fluid-containing vessel.
Description of the Related Art
Corrosion protection is required for steel structures that are exposed to corrosive fluids. The steel structures can be any structure exposed to corrosive fluids including, for example, a vessel that contains or is exposed to water or corrosive fluids. A protective coating on the steel or cathodic protection can be used to protect steel from corroding.
Cathodic protection for vessels is typically done with galvanic anodes in a technique known as galvanic anode corrosion protection (“GACP”). There are three types of galvanic anodes that are typically used for corrosion protection, namely, magnesium, aluminum and zinc anodes. The magnesium anodes often demonstrate high potential and, thus, corrode in less than one year in vessel protection. The aluminum anodes are also consumed rapidly, particularly when the temperature is more than 50° C. in the vessel. The normal zinc anodes are not consumed as quickly, but may reverse polarity at higher temperatures, meaning instead of acting as anodes, they may become the cathode at high temperature. That is why high temperature zinc (“HTZ”) anodes are often used in vessels at temperatures above 50° C. and up to 70° C. These GACP conventional anodes demonstrate undesirable properties when used in severe conditions. What is meant by severe here is a combination of low resistivity, high temperature and/or high H2S. The consumption rate of HTZ anodes, for example, is increased from 12 Kg/A-Y in normal conditions to 16 Kg/A-Y in severe conditions. That is 30% more, reflecting into 30% shorter anode life.
Another problematic issue in traditional cathodic protection systems is known as erosion corrosion. Where fluids are flowing past an exposed anode, the anode can deteriorate due to erosion from such flowing fluid.